Bonding electronic components by soldering has been done for years. The factors to be considered for creating a strong, enduring, non-corrosive physical and electrical bond between electronic components include: the type of solder used; the nature of the component material composition; the amount and duration of heat required; and the use of anti-oxidation measures such as fluxing. Soldering details vary considerably depending upon other factors such as the size and type of electronic components to be bonded.
With miniature electronic components, bonding technology has become increasingly important especially with the proliferation of microelectronic semiconductor laser diodes such as ridge wave-guide laser diodes.
Laser diodes are tiny devices typically measuring about 750 .mu.m in length along the optical resonator by 500 .mu.m in width across the mirror facets by 100 .mu.m in height from the positive surface (P-side) to the negative surface (N-side). Appreciable heating of the laser diodes occurs during operation, requiring heat sinking for improving operational consistency, preventing component damage, and elongating life expectancy.
One particular type of laser diode of interest is the index guided ridge wave-guide laser diode. In the ridge wave-guide structure, the top of the laser device is etched down to be very close to the active layer. Only a small part of the diode is not etched. This produces a type of plateau above the active layer, which ultimately becomes the lasing region. The evanescent field of propagation wave extends into the plateau region and is efficiently guided in the process. In this structure, the electrical confinement is provided by the opening in the insulating oxide layer. The wave-guides and active regions are typically located on the P-side of the laser diodes. In a typical striped geometry, the wave-guides may be about 3-5 .mu.m wide, 0.1-0.2 .mu.m thick and 750 .mu.m long. The active regions are isolated from one another by grooves coated with non-conductive dielectric layers. Most frequently, ridge wave-guide laser diodes are operated N-side down under low power conditions with a pumping current in the order of 100 mA and power dissipation of about 20 mW. Laser diodes of this type are typically soldered N-side down to a heat sink, i.e. the P-side surface containing the wave-guide grooves is mounted upwards. P-side up operation is conventional since low power operation of ridge wave-guide laser diodes is necessary for many applications, such as coupling optical fibers which would otherwise burn and fail if exposed to higher power laser diodes, and since handling the P-side down can easily cause damage to the laser diodes.
In contrast to the conventional N-side down soldering of ridge wave-guide laser diodes, a few applications require higher power emission in the order of 750 mW or more, which is accommodated by N-side up bonding of the laser diodes to the heat sink. By soldering the laser diodes P-side down to the heat sink, greater dissipation of operating heat from the laser diodes is achieved, allowing cooler operation while simultaneously sustaining a higher operating current. However, bonding the P-side to a heat sink by conventional soldering techniques typically results in incomplete bonding in the grooves of the ridge wave-guide laser diode. This shortcoming in turn results in: reduction in the strength of the bond; a less reliable electrical contact; operational inconsistency; and a loss in heat sinking capability; all of which have negative impacts on operational power.
One practical solution to the above shortcomings of conventional P-side down soldering is recognized in the prior art by filling the grooves of the ridge wave-guide laser diodes with polyimide before soldering so that the interface between the P-side of the laser diodes and the heat sink surface become coplanar while still maintaining proper isolation between active regions. However, filling the grooves with polyimide requires additional laser diode fabrication steps, as well as additional care when handling the polyimide and the diodes. Consequently, there is a need for improved methods for bonding heat sinks to the P-sides of laser diodes.
Accordingly, a primary object of the invention is to provide a method and apparatus for maximizing adhesion and bonding of solder between uneven surfaces to be soldered together.
Another object of the invention is to improve the life expectancy, increase and sustain operating power, and improve reliability of a ridge wave-guide laser diode by an improved method and apparatus for soldering the laser diode to a heat sink.
Yet another object of the invention is to provide increased heat transfer capability between a heat sink soldered to an electronic component when one or both of the surfaces to be soldered are irregular or otherwise not planar.
Other objects of the invention will, in part, appear hereinafter and, in part be easily derived from the following detailed description read in conjunction with the drawings.